National Centre for Nuclear Research Search for possible dark matter effects in leptonic decays of light mesons measured in the WASA-at-COSY experiment Marcin Berłowski National Centre for Nuclear Research Uppsala, Sweden 24.IV.2014 1

Outline Motivation WASA-at-COSY Experimental setup Results The future

Dark matter Seen in galaxy rotation curves and in gravitational lensing Not in the Standard Model Interacting by a gravitational force

Dark matter We are looking for a hypothetical U boson that could mediate the annihilation of dark matter particles: Simple extension of the Standard Model with U’ symmetry with ε coupling constant (also called mixing parameter ε2=α’/α) 511 keV line from around the Galaxy center measured by the INTEGRAL experiment The assumption that such a particle exists could provide an explanation for the results seen by many experiments (PAMELA, ATIC, …)‏ Mass U=10-200 MeV [Phys.Rev.D75:115017,2007]

Dark matter Vector boson (dark photon)‏ It’s signature should be visible in Dalitz decays of pseudoscalar mesons Two methods to explore: e+e- mass spectrum and photon mass spectrum in this decay

γcτ~1mm(γ/10)(10-4/ε)2·(100MeV/mU)‏ Dark matter Lifetime and decay length: γcτ~1mm(γ/10)(10-4/ε)2·(100MeV/mU)‏ Constrains on ε (coupling constant) vs mU space set by the results of many different experiments – astrophysics, beam-dump experiments, particle decays, and measurement of magnetic moments of leptons (g-2)‏ arXiv:1104.2747

WASA-at-COSY Collaboration 26 institutes from all around the world Nearly 200 people involved COoler SYnchrotron localized in Forshungszentrum Juelich, Germany

WASA-at-COSY experiment Storage ring with circumference of 184 m for protons and deuterons Maximum beam energy 3.7 GeV

WASA-at-COSY physics program Meson (π°, η, ω) decays and production – symmetry breaking, rare decays Search for physics beyond the Standard Model Resonance effects in multi pion production Eta-mesic nuclei Isospin violation process in dd→4Heπ°

WASA setup WASA – Wide Angle Shower Apparatus Three main parts: pellet target, central detector and forward detector

Forward detector

Central detector

Pellet target Frozen hydrogen or deuterium Frequency 8-10 kHz Droplet diameter 25-35 μm Velocities around 80 m/s COSY

Drift chamber 1738 tubes in 17 layers Diameter of tubes 4 to 8 mm Filled with CO2 i argon Magnetic field of 1 Tesla Working principle:

Electromagnetic calorimeter Covers most of the angular space 1012 CsI(Na) crystals varying from 20 to 30 cm (~16 X0)‏

Results

pp→ppπ° data Collected with energy 550 MeV (below threshold for π+π- production)‏ Two weeks of data taking (effective 4 days) Biggest sample of Dalitz decay events 5·105 π°→γe+e- (SINDRUM ~105)‏ We were looking for U→e⁺e⁻ in e+e- mass spectrum from π°→γe+e- decay The results were published in Phys. Lett. B 726 (2013), 187 Carl-Oscar Gullström & Uppsala Univ.

Dalitz decay of π° meson · Data --- MC sum --- MC π⁰ → e⁺e⁻γ (BR=1.2%)‏ --- MC external conversion --- MC π⁰ → e⁺e⁻γ plus false e⁺ from π⁺ „Search for a dark photon in π⁰ → e⁺e⁻γ decay” [arXiv:1304.0671]

ε2 coupling constant vs U boson mass „Search for a dark photon in π⁰ → e⁺e⁻γ decay” [arXiv:1304.0671]

 meson Mass ~550 MeV/c2 Big mass (in comparison to  mesons) connected to admixture of strange quarks Long lifetime due to the fact, that all of its decay channels are somehow forbidden

BRexp=(7.48±0.29±0.25)x10-8; BRtheo=(6.2±0.1)x10-8 Dark matter KTeV collaboration results showing 3.3σ deviation from a very precise theoretical calculations for a Branching Ratio for π°e+e- BRexp=(7.48±0.29±0.25)x10-8; BRtheo=(6.2±0.1)x10-8 Proposed explanation by a vector boson U coupling both to quarks and electrons Similar effect could be seen in ηe+e-, but here we have much smaller statistic [Nucl.Phys.B683:219,2004] [Phys.Rev.D78:115002,2008]

Leptonic decays of  meson Branching ratios: → 39% exp →e+e- ~0.7% exp →e+e- ~?10-9? theo We have some theoretical predictions within the frames of the Standard Model, but enhanced BR for the decay can be a sign of an unconventional process.

Previous analysis in CELSIUS/WASA

Collected data and trigger system Experiment in Autumn of 2008 using pp→pp reaction @ 1.4 GeV Proton run chosen because of higher  production rate During 2 weeks of data taking we collected ~150 million events Trigger system used a special reaction property demanding high energy deposits in both halves of electromagnetic calorimeter Trigger worked nearly the same for each leptonic decay channel Trigger simulation preliminary

Data pp→pp(→)‏ IMγγ MMpp Two photon events Used for normalization πº η Data MMpp η 25

→ preliminary ~5.9·107 η πº Numer of eta mesons produced: With systematic error of 15% 26

MCeeγ e+e- pairs in data e+e- pairs Data at the beginning of analysis The invariant mass of e+e- pairs produced by a virtual photon conversion is usually small MCeeγ Where MP is in this case η meson mass, q2 is Mee mass 27

Events with small e+e- mass pp→pp(→e+e-)‏ πº η η Data Events with small e+e- mass

Photon conversion in the beam pipe 60 mm Data preliminary MCeeγ MCγγ

Photon conversion in the beam pipe + Data --- MC η→γγ --- MC η→e⁺e⁻γ --- MC sum preliminary e+e- pairs mass, before and after conversion reduction conversion to Dalitz ratio - before: ~1:2, after: ~1:20 signal reduction after conversion reduction ~20%

→e+e- conclusions Good agreement between the number of observed Dalitz decays and the number of eta meson calculated from the normalization → channel It served as a testing field for developing needed analysis methods such as: identification and measurement of electrons understanding photon conversion in the beam pipe investigating the detector response for electrons with various energies

In search for →e+e- - background reactions pp→pp+- 100 times greater cross section than for  meson production Two charged particles in the final state →e+e- Photons with small energy can be undetected pp→p∆(1232)→pp(*→e+e-)‏ The same final state as in our reaction Other physical processes were found to be negligible when compared to those mentioned above

In search for →e+e- - reaction signature (simulations)‏ X MC pp→pp+- pp→pp(→e+e-)‏ An example how to distinguish between pions and electrons based on their energy deposits in the electromagnetic calorimeter

Analysis – final results The complete lack of signal events allowed us to set the BR limit for →e+e- equal to <2.1·10-6 CL90% If we take into account Poisson statistics for a small number of events and dominating systematical error from normalization the result transforms to the preliminary result: 3.9·10-6 with confidence level 90% (todays best limit in PDG 5.6·10-6 Eur.Phys.J. A48 (2012) 64)‏ Part of my PhD Thesis

Data: η Dalitz decay Data analysis (by Damian Pszczel) is in progress At least 10⁸ η mesons produced in pp @ 1.4 GeV Lower statistics than in π⁰ case but up to higher masses We expect around 40000 η Dalitz events in the full data set Small part of data

Summary and something about future Analysed sample of ~6·107  mesons and ~109 π° Several channels of  i π° decays involving leptons were observed and measured Better limit for BR of →e+e- was established as well as the limit for ε parameter for U boson The analysis performed in order to search U boson in π⁰→e⁺e⁻γ has been published, the article summarizing the search for →e+e- is in preparation Analysis of a bigger statistic (x8) of  meson decays and π⁰ (x4) is in progress

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